I don't think it's because the ps3 isn't capable. They could do persistant stat tracking without signing in on psn. All they would have to do is have you make an account on their black ops website, linking YOUR psn to YOUR black ops account (the one that gets tracked by the server) When you play splitscreen online you could sign in to your black ops account get your information from the server. Nothing too hard here, just ****ing microsoft buying out all that they can as a last resort before they start losing ground.
*edit*
After looking at the link
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it says,"
•You can connect a second controller and bring a guest online (i.e. “JD_2020” & “JD_2020(1)”

.
•Guests can rank up, earn unlocks, etc – all guest progress goes away once signed out."
We have the feature, but its on a GUEST ACCOUNT. that has been stated many times.